1. Field of the Invention
The present invention relates to a steering wheel, more particularly, to a steering wheel which has a pad portion formed integral with covers and which is furnished with an air bag device.
2. Description of the Related Art
Steering wheels generally comprise an annular ring portion, a pad portion provided in the center of the ring portion and a plurality of spoke portions extending from the ring portion toward the center of the steering wheel. Said ring portion comprises a core and its cover and, similarly, the spoke portions each comprise a core and its cover. The cores of the spoke portions are coupled at the distal end to a boss plate (boss), which is usually mounted on an end of the steering wheel by tightening with a nut.
The thus constructed steering wheel is such that the cores of the spoke portions and, hence, the boss which is an integral part of the main body of the steering wheel are mounted on the steering shaft by tightening with a nut and, subsequently, an air bag device and the like are installed and the pad portion is mounted from above.
In recent years, there was proposed a steering wheel which had the covers of the ring and spoke portions formed integral with the pad portion to eliminate the boundary between each of the covers and the pad portion (for example, Japanese Utility Model Publication No. Hei. 2-133955). According to this technology, the covers are made integral with the pad portion to achieve an improvement in the design of the steering wheel.
In spite of the improvement in design and the ensuring tendency toward the use of diverse external shapes, the steering wheel which has the covers formed integral with the pad portion suffers from the following problem during its mounting. Namely, the steering wheel which has the pad portion formed integral with the covers has no opening above the boss, thus making it impossible to place an air bag device and the like in position from above the boss.
On the other hand, the steering wheels are occasionally provided with leather members made of genuine or artificial leather which are installed typically on the surface of the core of the ring portion with a view to improving the wheel design. A common method of installing the leather member is by stitching its edge portions to the cover with a strand of high strength. Another approach that is also commonly employed is what is generally referred to as "closing"; according to this technique, grooves are formed in the cover of the ring portion and the edge portions of the leather member are fitted into the grooves so that it is installed in position.
However, this conventional art technique of "closing" has occasionally encountered a difficulty in forming sufficiently deep grooves in the cover of the ring portion on account of the core in said cover. This has caused the potential failure to secure the desired strength in the installation of the leather member.
In addition, the cover in which the grooves are formed is made of a soft resin material such as polyurethane foam, so it is easy to deform under external stress and has the disadvantage of suffering from deformation over time. This has caused the potential opening of the grooves during service. As a result, there has occurred the potential failure to secure the desired strength in the installation of the leather member, as well as the potential degradation in its external appearance and quality.